This invention relates to improvements in a surround sound processor. A surround sound processor operates to enhance a two-channel stereophonic source signal so as to drive a multiplicity of loudspeakers arranged to surround the listener, in a manner to provide a high-definition soundfield directly comparable to discrete multitrack sources in perceived performance. An illusion of space may thus be created enabling the listener to experience the fullness, directional quality and aural dimension or "spaciousness" of the original sound environment. The foregoing so-called periphonic reproduction of sound can be distinguished from the operation of conventional soundfield processors which rely on digitally generated time delay of audio signals to simulate reverberation or "ambience" associated with live sound events. These conventional systems do not directionally localize sounds based on information from the original performance space and the resulting reverberation characteristics are noticeably artificial.
To accomplish this end, a surround sound processor typically comprises an input matrix, a control voltage generator and a variable matrix circuit.
The input matrix usually provides for balance and level control of the input signals, generates normal and inverted polarity versions of the input signals, generates sum and difference signals, and in some cases generates phase shifted versions, and/or filters the signals into multiple frequency ranges as needed by the remainder of the processing requirements.
The control voltage generator includes a directional detector and a servologic circuit. The directional detector measures the correlations between the signals which represent sounds encoded at different directions in the stereophonic sound stage, generating voltages corresponding to the predominant sound directional location. The servologic circuit uses these signals to develop control voltages for varying the gain of voltage controlled amplifiers in the variable matrix circuit in accordance with the sound direction and the direction in which it is intended to reproduce the sound in the surrounding loudspeakers.
The variable matrix circuit includes voltage-controlled amplifiers and a separation matrix. The voltage controlled amplifiers amplify the input matrix audio signals with variable gain, for application to the separation matrix, where they are used to selectively cancel crosstalk into different loudspeaker feed signals. The separation matrix combines the outputs of the input matrix and of the voltage-controlled amplifiers in several different ways, each resulting in a loudspeaker feed signal, for a loudspeaker to be positioned in one of several different locations surrounding the listener. In each of these signals, certain signal components may be dynamically eliminated by the action of the detector, control voltage generator, voltage controlled amplifiers (VCA's) and separation matrix.
In his copending U.S. patent application No. 533,091, entitled "Surround Processor", Fosgate discloses a servologic circuit of a control voltage generator which employs a width modulated pulse train to vary the time constants applied to the control signals, in accordance with the difference between the raw detector output signals and the smoothed control signals resulting after they have been passed through the variable time-constant filters, thereby placing the modulating elements within a feedback loop, as shown in FIGS. 5-7 of the application and FIG. 5a herein.
In U.S. Pat. No. 4,932,059, Fosgate discloses use of variable time constants operated by means of a width-modulated pulse train, the duty cycle of which is controlled by means of a signal level detector, and further by means of a one-shot monostable multivibrator responsive to signal "attacks." This one-shot is so designed that the output pulse duration is sufficiently long to ensure that the control signals reach their appropriate values fairly quickly, but is sufficiently short that very low transient intermodulation distortion occurs, such that the listener is unable to hear any artifacts in the decoding process. This scheme is described with reference to FIGS. 2-5 and 6a and 6b of the above-referenced patent and in FIG. 5b herein.
In surround sound processors, much of the subtleties of the presentation are due to the characteristics of the directional detector and servologic circuit of the control voltage generator and of the VCA's. As these are further refined, the apparent performance becomes more transparent and effortless sounding to the listener.